PRINCIPLE OF AERODYNAMICS

What makes a paper airplane fly? Air — the stuff that's all around you. Hold your hand in front of your body with your palm facing sideways so that your thumb is on top and your pinkie is facing the floor. Swing your hand back and forth. Do you feel the air? Now turn your palm so it is parallel to the ground and swing it back and forth again, like you're slicing it through the air. You can still feel the air, but your hand is able to move through it more smoothly than when your hand was turned up at a right angle. How easily an airplane moves through the air, or its aerodynamics, is the first consideration in making an airplane fly for a long distance.

Drag & Gravity
Planes that push a lot of air, like your hand did when it was facing the side, are said to have a lot of "drag," or resistance, to moving through the air. If you want your plane to fly as far as possible, you want a plane with as little drag as possible. A second force that planes need to overcome is "gravity." You need to keep your plane's weight to a minimum to help fight against gravity's pull to the ground.

Thrust & Lift
"Thrust" and "lift" are two other forces that help your plane make a long flight. Thrust is the forward movement of the plane. The initial thrust comes from the muscles of the "pilot" as the paper airplane is launched. After this, paper airplanes are really gliders, converting altitude to forward motion.

Lift comes when the air below the airplane wing is pushing up harder than the air above it is pushing down. It is this difference in pressure that enables the plane to fly. Pressure can be reduced on a wing's surface by making the air move over it more quickly. The wings of a plane are curved so that the air moves more quickly over the top of the wing, resulting in an upward push, or lift, on the wing.

The Four Forces in Balance
Long flights come when these four forces — drag, gravity, thrust, and lift — are balanced. Some planes (like darts) are meant to be thrown with a lot of force. Because darts don't have a lot of drag and lift, they depend on extra thrust to overcome gravity. Long distance fliers are often built with this same design. Planes that are built to spend a long time in the air usually have a lot of lift but little thrust. These planes fly a slow and gentle flight.

Materials

Paper (copier or computer printer paper works best), some 1/2" or 3/4" cellophane tape, and a paper clip. (make a copy of the "Bernoulli Plane" template so you can make several and experiment with different ways of making it fly).

Procedure

Build a special paper airplane to demonstrate how and why airplanes and most birds can fly (when they're not flapping their wings).

Click the paper airplane image below to see and print the paper airplane template

Cut out the airplane's shape along the dark solid line. Next, fold the top half at the dotted line so that it meets the bottom half. Don't, however, fully crease the paper at the fold (we want to make a nice 'tear drop' air foil shape). After folding the paper back, put a small piece of tape at the wing tips and at the center at the points marked A, B, and C. Now fold the plane along the center crease so that it creates a flattened out 'V'. The angle of the 'V' should be no more than about 15 degrees.

Test fly the plane and adjust its stability. Keep the nose of the plane from rising (stalling) by adding a small weight to the nose (point D), a paper clip or two does nicely. You can also adjust how much the plane dives or climbs by cutting small slots in the tail of the plane and bending the paper at the cuts up or down. Experiment with putting them up or down and seeing what effect that has on the way the plane flies. People who know about airplanes call these little 'tabs' an *elevator if it makes the plane go up or down and a **rudder if it makes the plane turn right or left.
   
What's going on?

Contrary to popular belief, airplanes don't float on the air, they're sucked up into it. This reason is known as Bernoulli's Principle. It says: "...as air travels faster [than surrounding air] across a surface, the air pressure against it is reduced...".

By curving the top of an airplane's wing, air above it has to travel farther (as the distance is greater) than the air below, forcing the air to move faster. The result is lower pressure on top and more pressure on the bottom. Another name for this is lift. The higher pressure below the wing is just like someone pushing from below the wing; the lower pressure above the wing is like someone pulling it up (like sucking on a straw to draw up milk in a glass). If a wing has enough lift upwards, it moves upward, if a wing has lift downwards, it moves downward.

Even though most paper airplanes have 'flat' wings, they still cause the air to move the same way. The plane that we built, the "Bernoulli Plane" has a real airfoil and more closely resembles and flies the way that real planes and most birds do

Things to Remember

Bernoulli's Principle is a relation discovered by the 18th-century Swiss scientist and mathematician Daniel Bernoulli. He discovered that the faster a fluid (such as air) moves, the lower is the pressure that it exerts.

*Elevator - Elevator is the term used to describe a plane's horizontal control surface on the tail. This surface enables a plane to pitch upwards or downwards. When an elevator surface moves upwards, the tail moves downwards (the nose of the plane then points up) and vice-versa. Without an elevator, it is hard to control the altitude of a plane as you can't control the rise and fall of the nose of the plane. The German word for elevator is Hohenruder [High-rudder].

**Rudder - This is the term used to describe the part of the tail that moves back and forth. This movement causes the tail of a plane to move which then turns the plane. There are lots of planes that have only rudder and elevator for turning right and left and moving upwards and downwards. Then there are others that do fine with just ailerons and elevator. Then there are planes that have everything. These are known as Full House sailplanes. (The German word for rudder is Ruder.)

Article adapted from website by Randy Carr